Control register

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A control register is a processor register which changes or controls the general behavior of a CPU or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, and coprocessor control.

Control registers in x86 series[edit]

CR0[edit]

The CR0 register is 32 bits long on the 386 and higher processors. On x86-64 processors in long mode, it (and the other control registers) is 64 bits long. CR0 has various control flags that modify the basic operation of the processor.

Bit Name Full Name Description
31 PG Paging If 1, enable paging and use the CR3 register, else disable paging
30 CD Cache disable Globally enables/disable the memory cache
29 NW Not-write through Globally enables/disable write-through caching
18 AM Alignment mask Alignment check enabled if AM set, AC flag (in EFLAGS register) set, and privilege level is 3
16 WP Write protect Determines whether the CPU can write to pages marked read-only when privilege level is 0
5 NE Numeric error Enable internal x87 floating point error reporting when set, else enables PC style x87 error detection
4 ET Extension type On the 386, it allowed to specify whether the external math coprocessor was an 80287 or 80387
3 TS Task switched Allows saving x87 task context upon a task switch only after x87 instruction used
2 EM Emulation If set, no x87 floating point unit present, if clear, x87 FPU present
1 MP Monitor co-processor Controls interaction of WAIT/FWAIT instructions with TS flag in CR0
0 PE Protected Mode Enable If 1, system is in protected mode, else system is in real mode

CR1[edit]

Reserved

CR2[edit]

Contains a value called Page Fault Linear Address (PFLA). When a page fault occurs, the address the program attempted to access is stored in the CR2 register.

CR3[edit]

Typical use of CR3 in address translation with 4 KiB pages.

Used when virtual addressing is enabled, hence when the PG bit is set in CR0. CR3 enables the processor to translate linear addresses into physical addresses by locating the page directory and page tables for the current task. Typically, the upper 20 bits of CR3 become the page directory base register (PDBR), which stores the physical address of the first page directory entry.

CR4[edit]

Used in protected mode to control operations such as virtual-8086 support, enabling I/O breakpoints, page size extension and machine check exceptions.

Bit Name Full Name Description
21 SMAP Supervisor Mode Access Protection Enable If set, access of data in a higher ring generates a fault[1]
20 SMEP Supervisor Mode Execution Protection Enable If set, execution of code in a higher ring generates a fault
18 OSXSAVE XSAVE and Processor Extended States Enable
17 PCIDE PCID Enable If set, enables process-context identifiers (PCIDs).
14 SMXE Safer Mode Extensions Enable see Trusted Execution Technology (TXT)
13 VMXE Virtual Machine Extensions Enable see Intel VT-x
10 OSXMMEXCPT Operating System Support for Unmasked SIMD Floating-Point Exceptions If set, enables unmasked SSE exceptions.
9 OSFXSR Operating system support for FXSAVE and FXRSTOR instructions If set, enables SSE instructions and fast FPU save & restore
8 PCE Performance-Monitoring Counter enable If set, RDPMC can be executed at any privilege level, else RDPMC can only be used in ring 0.
7 PGE Page Global Enabled If set, address translations (PDE or PTE records) may be shared between address spaces.
6 MCE Machine Check Exception If set, enables machine check interrupts to occur.
5 PAE Physical Address Extension If set, changes page table layout to translate 32-bit virtual addresses into extended 36-bit physical addresses.
4 PSE Page Size Extension If unset, page size is 4 KiB, else page size is increased to 4 MiB (or 2 MiB with PAE set).
3 DE Debugging Extensions If set, enables debug register based breaks on I/O space access
2 TSD Time Stamp Disable If set, RDTSC instruction can only be executed when in ring 0, otherwise RDTSC can be used at any privilege level.
1 PVI Protected-mode Virtual Interrupts If set, enables support for the virtual interrupt flag (VIF) in protected mode.
0 VME Virtual 8086 Mode Extensions If set, enables support for the virtual interrupt flag (VIF) in virtual-8086 mode.

Additional Control registers in x86-64 series[edit]

EFER[edit]

Extended Feature Enable Register (EFER) is a model-specific register added in the AMD K6 processor, to allow enabling the SYSCALL/SYSRET instruction, and later for entering and exiting long mode. This register becomes architectural in AMD64 and has been adopted by Intel. Its MSR number is 0xC0000080.

Bit Purpose
63:16 Reserved
15 TCE (Translation Cache Extension)
14 FFXSR (Fast FXSAVE/FXRSTOR)
13 LMSLE (Long Mode Segment Limit Enable)
12 SVME (Secure Virtual Machine Enable)
11 NXE (No-Execute Enable)
10 LMA (Long Mode Active)
9 Reserved
8 LME (Long Mode Enable)
7:1 Reserved
0 SCE (System Call Extensions)

CR8[edit]

CR8 is a new register accessible in 64-bit mode using the REX prefix. CR8 is used to prioritize external interrupts and is referred to as the task-priority register (TPR).[2]

The AMD64 architecture allows software to define up to 15 external interrupt-priority classes. Priority classes are numbered from 1 to 15, with priority-class 1 being the lowest and priority-class 15 the highest. CR8 uses the four low-order bits for specifying a task priority and the remaining 60 bits are reserved and must be written with zeros.

System software can use the TPR register to temporarily block low-priority interrupts from interrupting a high-priority task. This is accomplished by loading TPR with a value corresponding to the highest-priority interrupt that is to be blocked. For example, loading TPR with a value of 9 (1001b) blocks all interrupts with a priority class of 9 or less, while allowing all interrupts with a priority class of 10 or more to be recognized. Loading TPR with 0 enables all external interrupts. Loading TPR with 15 (1111b) disables all external interrupts.

The TPR is cleared to 0 on reset.

See also[edit]

References[edit]

  1. ^ Anvin, H. Peter. "x86: Supervisor Mode Access Prevention". Retrieved 15 April 2013. 
  2. ^ "AMD64 Architecture Programmer?s Manual Volume 2: System Programming". AMD. Retrieved 3 April 2013. 

External links[edit]